A technology that produces a molded article having a desired shape is known in which a molding material comprising a bundle-like aggregate of discontinuous reinforcing fibers (for example, carbon fibers) (hereinafter, also referred to as a fiber bundle) and a matrix resin is used and it is molded by heating and pressurizing. In such a molding material, a molding material comprising a fiber bundle having a large number of single fibers is excellent in flowability at the time of molding, but tends to be inferior in mechanical properties of a molded article. On the other hand, a fiber bundle adjusted to an arbitrary number of single fibers is used as a fiber bundle in the molding material, aiming to satisfy both flowability at the time of molding and mechanical properties of the molded article.
As a method of adjusting the number of single fibers of the fiber bundle, for example, JP-A-2002-255448 and JP-A-2004-100132 disclose methods of performing a separation processing using a plurality of fiber bundle winding bodies prepared by winding a plurality of fiber bundles in advance. In those methods, however, because the number of single fibers of each fiber bundle treated in advance is restricted, the adjustment range is limited and, therefore, it is difficult to adjust to a desired number of single fibers.
Further, for example, JP-A-2013-49208, JP-A-2014-30913 and Japanese Patent No. 5512908 disclose methods of longitudinally slitting a fiber bundle to a desired number of single fibers by using disk-shaped rotary blades. In those methods, although it is possible to adjust the number of single fibers by changing the pitch of the rotary blades, since the fiber bundle longitudinally slit over the entire length in the lengthwise direction has no convergence property, the yarn after the longitudinal slit tends to become difficult in handling such as winding it on a bobbin or unwinding the fiber bundle from the bobbin. In addition, when conveying the fiber bundle after the longitudinal slitting, the split end-like fiber bundle generated by the longitudinal slit may be wrapped around a guide roll, a feed roll or the like, which may not be easy to convey.
Further, WO 2012/105080 discloses a method of cutting a fiber bundle to a predetermined length at the same time as a longitudinal slit by a separation cutter having a lateral blade perpendicular to the fiber direction in addition to a longitudinal blade having a longitudinal slit function in a direction parallel to the fiber direction. According to that method, it becomes unnecessary to once wind the fiber bundle after the longitudinal slit to the bobbin and transport it, and the handling property is improved. However, since the separation cutter has the longitudinal blade and the lateral blade, when one of the blades reaches the cutting life first, an obstacle arises that the entire blade has to be exchanged.
Further, for example, JP-A-2011-241494 and US Patent Publication No. 2012/0213997 A1 describe a method in which a roll having a plurality of projections is provided on the outer circumferential surface of the roll, and the projections of the roll is pushed into a fiber bundle to partially separate the fiber bundle. In that method, however, because the circumferential speed of the roll and the conveying speed of the fiber bundle are basically the same speed synchronized with each other, it is impossible to control the lengths and the like of the separated-processed section and the not-separated-processed section, and it is difficult to obtain a partially separated fiber bundle with an optimum form.
Furthermore, EP-A-2687356 A1 describes a special method of forming intermittently extending flow paths to facilitate resin impregnation in a fiber bundle by a monofilament extending in a direction orthogonal to the fiber bundle. However, that manner relates to a technology of forming a flow path to facilitate resin impregnation in a fiber bundle and, therefore, it is basically a technology different from separation of a fiber bundle such as large tow.
As described above, to satisfy both the flowability during molding and the mechanical properties of a molded article, a fiber bundle adjusted to an arbitrary optimum number of single fibers is required.
Furthermore, in passing through the above-described longitudinal slitting process at a state where a fiber bundle is twisted such as twist exists in the fiber bundle itself or twist occurs during traveling of the fiber bundle at the separation step, because crossing fiber bundles are cut in the lengthwise direction, a problem occurs in that the fiber bundle is cut at a small length before and after the longitudinal slitting process and the longitudinal slitting cannot be continuously performed.
Accordingly, it could be helpful to provide a partially separated fiber bundle capable of forming a fiber bundle with an optimum number of single fibers to manufacture a molding material used for molding a composite material, and a method of producing a partially separated fiber bundle capable of slitting continuously and stably into the optimum fiber bundle formation. In particular, it could be helpful to provide a partially separated fiber bundle capable of controlling to an optimal distribution state of thin fiber bundles and thick fiber bundles when cutting/spraying the partially separated fiber bundle and preparing an intermediate base material of fiber bundles of discontinuous fibers to manufacture a molding material used for molding a composite material, thereby exhibiting the flowability during molding and the mechanical properties of a molded article at a good balance, and a method of producing the same. Furthermore, it could be helpful to provide a partially separated fiber bundle and a method of producing the same, enabling a continuous slitting without concerning about the exchange life of a rotary blade even in case of a fiber bundle including twist or a fiber bundle of a large tow having a large number of single fibers.
Further, it could be helpful to provide a fiber-reinforced resin molding material in which the above-described partially separated fiber bundle is matted and impregnated with a resin, and a method of producing a fiber-reinforced resin molding material having a series of steps up to manufacture it.